With ς a labial stop forms ψ, a palatal stop forms ξ.
|λείψω ||κῆρυξ |
|τρί_ψω ||ἄξω |
|γράψω ||βήξ |
a. The only stop that can stand before ς is π or κ, hence β, φ become π, and γ, χ become κ. Thus, γραφ-σω, ἀγ-σω become γραπ-σω, ἀκ-σω.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].